The invention relates to a method and circuit for stopping the test run of a rotor being balanced, and more particularly to a method and circuit for stopping the test run of a rotor being balanced using an electromagnetic brake.
In conventional wheel balancing machines for measuring the imbalance of rotors, in particular in relation to motor vehicle wheels, the test run of the rotor to be balanced, is stopped with the aid of an electromagnetic brake, which is provided with a magnetic element having a potted coil. The brake also includes a friction lining secured to the magnetic element or stator and an armature element comprising an armature disk, which is subjected to a spring force so that an air gap is provided between the magnetic element and the armature element when the brake is released. The armature element is connected to that part of the machine, which must be stopped, for example a motor shaft of an electric motor for driving the primary shaft of the balancing machine. The air gap is adjusted to a rated size. When a direct current is supplied, to the magnetic element the armature element is pulled toward the friction lining of the magnetic element by the resultant magnetic field produced by the electric current. Braking is then done at the rated braking moment by means of frictional engagement of the armature disk and the friction lining. Once the voltage producing the direct current is shut off, the spring force, which is transmitted by a pre-stressed spring to the armature disk, retracts the armature disk to its original position. When attracting a clamping device, such as a tensioning nut, it is necessary when the rotor is clamped to the primary shaft, for the electromagnet brake to act as a holding brake, to attain satisfactory centering and a secure seat of the rotor on the primary shaft of the balancing machine. This dictates a relatively high rated braking moment, and a correspondingly high rated voltage for applying the armature disk to the friction lining. When the rated voltage is switched on in order to brake the test run, this produces an excessively high braking moment.